In recent years, the demand for a valve nozzle for injection molding has increased during development of the automation of a molding process.
Conventionally, this kind of valve nozzle for injection molding is such constructed as shown in FIG. 5, which is disclosed in Japanese Patent Laid-open (Kokai) No. 163542/82. According to such prior art, a shut-off pin 4 slidably sustained on a torpedo 3a, 3b is biased toward a gate 2 formed in the front end of a nozzle head 1 by a spring 5 for closing a resin passage 6. When resins are supplied from the side of an adapter 7 and the pressure of the resins becomes greater than a certain extent or level, the spring 5 yields permitting rearward movement of the shut-off pin 4 for opening the gate 2.
In the above-mentioned structure, however, melted and highly-pressurized resins approximately at 1000 kg/cm.sup.2 may flow into the gap between the torpedo 3a and the shut-off pin 4 and remain therein for a long time, impeding sliding of the shut-off pin 4 due to deterioration and dissolution of such remaining resins. Thus, there exists a problem that the gap should be formed with a high degree of accuracy in the order of about 1 .mu.m for prevention of the entry of resins.
There also remains a problem that even when the gap is defined more accurately, resins can still continue to gradually penetrate into the gap. Resins enclosed within the gap are not allowed to be displaced anywhere so the that resins deteriorate and dissolve, thereby interfering with the sliding movement of the shut-off pin. The expected life span of the valve nozzle tends to be thus shortened.